Hi, I have seen several times online examples of wallets where lined leather isn't stitched, but simply glued on the open side of a pocket. An example can be seen here: https://www.reddit.com/gallery/j0w0wd You can see the pink lining is simply glued on that exposed edge to the main leather.
I think in this example edge paint may have been used. But I have seen examples without edge paint. Without the edge paint wouldn't the layers risk splitting apart?
Could simple glue and burnishing hold 2 layers of leather together reliably?
What Phil Said.
I will only add that, to sum everything up, you just need to think about the use the pieces of leather glued will be subjected to.
Quick example: the card holder you shared. If the craftsman used a water based glue, there is a chance that if the owner puts the card holder in his pocket and lives in very hot place, the leather will get damp (My cardholder in my pocket right now is damp for example) and the glue might give up and stop "gluing".